1. Field of the Invention
The present invention relates to a quinoxaline derivative, a light-emitting element using the quinoxaline derivative, and a light-emitting device and an electronic appliance including the light-emitting element. More specifically, the present invention relates to a quinoxaline derivative having an excellent electron-transporting property, a light-emitting element using the quinoxaline derivative, and a light-emitting device and an electronic appliance including the light-emitting element.
2. Description of the Related Art
An organic compound has various material systems compared to an inorganic compound, and has possibility to synthesize materials having various functions depending on the molecular design. Owing to these advantages, electronics (particularly, photo electronics) which uses a functional organic material has been attracting attention in recent years. For example, a solar battery, a light-emitting element, an organic transistor, and the like can be mentioned as examples of an electronic device using an organic compound as a functional organic material. These are devices taking advantage of electric properties and optical properties of the organic compound. Among them, in particular, a light-emitting element has been developing remarkably.
Such a light-emitting element which is thin, lightweight, and capable of fast response is expected to be applied to a next-generation flat panel display. In addition, it is said that a light-emitting device in which light-emitting elements are arranged in matrix has advantages over the conventional liquid crystal display devices in that the viewing angle is wide and visibility is high.
A light-emitting element has a structure in which a layer including a light-emitting substance is interposed between a pair of electrodes (anode and cathode). It is said that, in such a light-emitting element, when a voltage is applied between a pair of electrodes, electrons injected from a cathode and holes injected from an anode are recombined in the layer including a light-emitting substance so as to form an exciton of the light-emitting substance, and light is emitted when the exciton returns to a ground state.
Such a light-emitting element has a lot of problems in materials in the case where an element property thereof is intended to be improved. In order to solve the problems, improvement of an element structure, development of materials, and the like have been carried out. In particular, an organic semiconductor material is inherently a hole-transporting material in many cases. Thus, currently, hole mobility of a hole-transporting material is higher than electron mobility of an electron-transporting material by several digits. Therefore, an electron-transporting material having an excellent electron-transporting property is desired to be obtained.
A light-emitting element has been reported, which uses a quinoxaline derivative as such a material excellent in electron-transporting property (Patent Document 1: Japanese Published Patent Application No. H9-13025). However, the light-emitting element has high drive voltage and is not yet perfect. Accordingly, various researches have been made in order to improve an electron-transporting property in a light-emitting element (Patent Document 2: Japanese Published Patent Application No. 2002-100479).